These molecules are also known
as coenzyme Q or mitoquinones. They are involved in electron transport in mitochondrial
preparations playing an essential role in the oxidation of succinate or NADH via the
cytochrome system. They serves not only as a coenzyme but also, in their reduced forms, as
antioxidants. They are synthesized de novo in all animal tissues and cannot thus be
regarded as vitamins. Ubiquinones are present in all aerobic organisms, plants, animals (the name ubiquinone was proposed with reference to their ubiquitous
occurrence) and bacteria, but are absent from Gram-positive eubacteria and the
archaebacteria. They were discovered by the Morton's group (Biochem J 1955, 59,
558) in
animal fat but their quinonoid structure was revealed by Crane two years later (Biochim
Biophys Acta 1957, 25, 220) in extracts from beef heart mitochondria. The compound had a
2,3-dimethoxy-5-methylbenzoquinone nucleus and a side chain of 10 isoprenoid units and was referred to as
coenzyme Q 10 . Later, homologues with 6, 7, 8 and 9 units were isolated from other
organisms, bacteria or higher organisms. The main form in man has 10 units but in rat has
9 units. Another system of nomenclature is used: ubiquinone(x) in which x designates the
total number of carbon atoms in the side chain, it can be a multiple of 5. Thus, coenzyme
Q10 is also named ubiquinone (50). Ubiquinone became the "official" name of the
compound in 1975 (IUPAC-IUB commission).
Recent review: "Biochemical, physiological and medical aspects of ubiquinone
function" by Ernster L et al. (Biochim Biophys Acta 1995, 1271, 195).

Ubiquinones accept one electron and are transformed into semiquinone
radicals (UQH°) or two electrons to give ubiquinol (UQH2)

Coenzyme Q is reducible by sodium dithionite or borohydride to its
hydroquinone form, and can in turn be reoxidized to the quinone by Ag2O or more slowly by
oxygen. The absorption spectra of the two forms are shown below. The quinone form has a
strong absorption band at 275 nm which disappears in the reduced form.

Ubiquinone-9: MW 794
Ubiquinone-10: MW862
Ubiquinones are readily destroyed by heating in alkali , but in the presence of pyrogallol
the destruction is minimized.